Deflection signal correction system including an analog multiplier

ABSTRACT

A system for correcting for inaccuracies in the positioning of a cathode-ray tube beam which inaccuracies may result from various factors including (1) curvature of the screen and (2) off-axis displacement of the electron gun. Unique analog multiplication means are disclosed for developing corrected deflection signals in response to position command signals in order to compensate for distortions which would otherwise be introduced due to the tube geometry.

United States Patent Inventor Martin C. Henderson Canoga Park, Calif.Appl. No. 870,853 Filed June 30, 1969 Division of Ser. No. 770,901, N0v.29, 1968 Patented Oct. 19, 1971 Assignee The Bunker-Ramo Corporation OakBrook, 11].

DEFLECTION SIGNAL CORRECTION SYSTEM INCLUDING AN ANALOG MULTIPLIER 5Claims, 2 Drawing Figs.

US. Cl 235/ 194, 307/229, 328/160, 235/198, 340/324 A, 315/18 Int. Cl606g 7/16 Field of Search 235/197,

References Cited UNITED STATES PATENTS 3,241,078 3/1966 Jones 307/229 X3,422,305 1/1969 Infante 1 315/24 3,432,650 3/1969 Thompson 307/229 X3,465,137 12/1966 Brouillette, Jr, et a1. 315/24 X 3,512,096 5/1970Nagata et a1 330/30DX Primary Examiner.loseph F. RuggieroAttorneyFrederick M. Arbuckle ABSTRACT: A system for correcting forinaccuracies in the positioning of a cathode-ray tube beam whichinaccuracies may result from various factors including (1) curvature ofthe screen and (2) off-axis displacement of the electron gun. Uniqueanalog multiplication means are disclosed for developing correcteddeflection signals in response to position command signals in order tocompensate for distortions which would otherwise be introduced due tothe tube geometry.

ANALOG some 5 1-1 I wI -I m K- PAIENTEDHEI 19 I971.

SHEET 10F 2 AT T OHN [T Y?) PAIENTEDum 19 I911 SHEET 2 [IF 2 ANALOG FIG.2

INl/Iz'N'l'OR. MARTIN C1 HE NDE HSON ATTORNEYS DEFLECTION SIGNALCORRECTIONSYSTEM INCLUDING AN ANALOG MULTIPLIER This is a division ofapplication, Ser. No. 779,901 filed Nov. 29, 1968.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates generally to display apparatus, as for example of thecathode-ray tube (CRT) type, and more particularly to means forcorrecting for inaccuracies in the positioning of a CRT beam.

In uncorrected CRT display systems, the coordinates of points displayedon the CRT screen do not precisely correspond to the intendedcoordinates as expressed by position command signals. A number ofidentifiable factors contributing to this distortion include thefollowing:

1. Deflection yokes tend to produce angular deflection which increasesfaster than uncorrected deflection currents for large deflections;portion or 2. The shape of the CRT screen normally introduces distortioninasmuch as the smaller the radius of screen curvature, the smaller thelinear deflection produced for a given angular deflection;

3. The electron gun maybe displaced from the axis of the screen. It may,for instance, be deliberately displaced from the axis to provideclearance for an optical port on the screen axis;

4. The beam may not be aimed accurately at the center of the displayarea; and

5. The deflection coils may not be precisely displaced by 90.

Theoretical analysis of these distortions indicate several components.The'principal error components in the X-axis caused by theaforementioned yoke deflection factor (1) are proportional to X and WK.Higher order components are proportional to X, X Y' Y X, etc.

An electron gun displaced from the X-axis will primarily introduceerrors related to XY in the X-direction and related to Y in theY-direction. The relative magnitude and sense of the errors depend onthe direction and magnitude of gun displacement. The gun displacementfactor also influences the magnitude of the error components introducedby the other distortion-causing factors. Nonperpendicular coils [factor(5) introduce Y-related errors into the X-deflection. Correspondingerror components will be introduced into the Y-deflection.

To summarize, distortion of a CRT pattern occurs as a result of severalcauses. The principal distortion components in X are proportional to Y,XY, X, Y, X", YX. Principal distortion components in Y are proportionalto X, XY, X Y Y and X Y. Higher order components are undoubtedly presentbut typically are not readily observed.

2. Description of the Prior Art In many known prior art systems,attempts have been made to correct for various positioning distortions.For example, some such systems have incorporated means for producingmagnetic fields in the drift space of the CRT following the deflectioncoils. This technique permits a fair correction to be achieved forcertain types of distortion. However, the correction is achieved at theexpense of added nonlinearity.

A further correction technique is disclosed in U.S. Pat. application,Ser. No. 610,614, filed Jan. 20, 1967 by Carl A. Eggert and assigned tothe assignee of the present invention. The system disclosed therein usesan analog multiplier circuit to produce a principal XY correction to theX-signal in a CRT having the electron gun displaced in the Y-directiononly, together with nonlinear diode resistor and absolute value circuitsto produce compensations for other observable distortionsand forimperfections in the multiplier.

SUMMARY OF THE INVENTION The present invention is directed to animproved system responsive to position command signals for developingdeflection signals corrected so as to compensate for various distortionsintroduced as a consequence of several diverse factors.

signals are developed by summing various correction signal In accordancewith the invention, the corrected deflection components produced by amultiplication process involving the position command signals.

in' accordance with a first aspect of the invention, variable means areprovided to permit individual adjustment of various correction signalcomponents to thus enable each significant distortion component to becompensated for and substantially eliminated. The number of correctionsignal components used depends upon the precision desired. Anycorrection term of the form KX"'Y" may be synthesized and added into theappropriate position command signal to develop a corrected deflectionsignal. 4

In accordance with a further significant aspect of the invention, uniquecircuitry is provided for performing analog multiplication. Briefly, inaccordance with this aspect of the invention, the product of two analogquantities is developed by interconnecting first and second differentialcircuits such that the distribution ratio is the same in the secondcircuit as it is in the first circuit and by causing the first analogquantity to determine the distribution ratio between the paths of thefirst circuit and the second quantity to determine the total flow in thesecond circuit whereby the quantitative difference between the flows inthe second circuit will represent the product of the analog quantities.

Although the display system embodiment disclosed herein specificallyprovides only for the correction of major horizontal and verticalposition command signals, it will be readily appreciated by thoseskilled in the art, that the teachings of the invention can be extendedto correct minor positioning signals as well.

DESCRIPTION OF THE FIGURES FIG. 1 is a schematic block diagram of asystem for correcting positioning signals in a CRT display system; and

FIG. 2 is a schematic diagram of an analog multiplier constructed inaccordance with the present invention.

It has been pointed out that the coordinates of points displayed on aCRT screen will not correspond precisely to the intended coordinate asexpressed by position command signals, unless some type of correction isintroduced. In accordance with the present invention, a correctionapparatus as illustrated in FIG. 1, is interposed between a source ofhorizontal (X) and vertical (Y) position command signals and the CRTdeflection means to compensate for various factors tending to introducebeam position distortion.

The beam position distortion eliminated by embodiments of the presentinvention is attributable to many different factors. For example, theshape of the CRT screen normally introduces distortion since its radiusof curvature is usually different from the distance between the beamdeflection means and the tube screen. This distortion is usuallyreferred to as pincushion distortion and is explained in somewhatgreater detail in the aforecited patent application.

Beam position distortion may also be caused, for example, by theelectron gun being displaced from the axis of the tube screen. Thisoff-axis displacement may be deliberate in order, for example, toprovide for an optical port in the tube envelope on the screen axis. Thedistortion attributable to the gun being off-axis is sometimes referredto as keystone distortion and is also described in somewhat greaterdetail in the aforecited patent application.

Beam position distortion is also attributable to many other factors.However, regardless of the particular distortion factors prevalent in aparticular CRT, it has been found that the magnitude of the beamposition distortion at any position on the CRT screen is related to theX- and Y-coordinates at that position. More particularly, the magnitudeof beam position distortion at any coordinate can be expressed in theform KX 'Y".

In accordance with the present invention, correction apparatus isprovided for interposition between a source of position command signalsand the CRT deflection means to add (or subtract) correction terms tothe position command signals to develop deflection signals forapplication to the tube deflection means. The number of correction termssuperimposed on the position command signals depends upon the precisiondesired. In accordance with the invention, any correction term of theform KX'Y" may be synthesized and combined with the position commandsignal to develop a corrected deflection signal.

In accordance with the preferred embodiment of the invention,illustrated in FIG. 1, the following distortion components in X arecompensated for: Y, XY, X, Y, X, and YX. The following distortioncomponents 2, Y are compensated for: X, XY, X, Y, Y, and XY. Althoughthe teachings of the present invention can be easily extended tocompensate for higher order distortion components, the distortionattributable thereto is not usually visually observable.

Attention is now called to FIG. 1 of the drawings which illustrates apreferred embodiment of the invention for correcting X- and Y-beamposition command signals respectively provided by sources and 12 todevelop corrected X- and Y- deflection signals for application todeflection means 14 and 16 associated with CRT 18. The CRT 18illustrated in FIG. 1 is of the type discussed in the aforecited patentapplication wherein the electron gun 20 is displaced from the X-axis ofscreen 22 in order to enable an optical port 24 to be formed in the tubeenvelope on the screen axis. The optical port 24 enables images to beoptically projected onto the screen 22 from the rear.

Prior to proceeding with a detailed discussion of the apparatusillustrated in FIG. 1, it is pointed out that the illustrated embodimentwas designed specifically to compensate for the principal distortionfactors present in a CRT of the type illustrated in FIG. 1 in which theelectron gun is displaced from the X-axis and symmetric with respect tothe Y-axis. Inasmuch as different types of distortion will be present indifferent tubes, it is recognized that it may be necessary to somewhatmodify the embodiment of FIG. 1 for use with different tubes. Regardlessof the tube type, however, it is emphasized that embodiments of theinvention are able to synthesize any correction terms of the form KX"Y"and it is accordingly pointed out that embodiments of the invention areuseful with any CRT to eliminate the principal distortion factorsencountered therein.

In considering the embodiment of FIG. 1 in detail, initial attentionwill be paid to the correction terms superimposed upon the X-positioncommand signal provided by source 10. The signal X is applied to firstand second input terminals 30 and 32 of an analog multiplier 34 to bedescribed in greater detail in conjunction with FIG. 2. The multiplier34 forms a product of the analog signals supplied to the input terminals30 and 32 thereof and, as will be seen hereinafter, represents theproduct as the difference between the currents flowing through outputterminals 36 and 38. Terminals 36 and 38 are connected to the inputterminals of an operational amplifier 40 which is responsive to thedifference between the currents through terminals 36 and 38, to developa signal X at its output terminal. As with all operational amplifiers, afeedback resistor 42 is connected between the amplifier output terminaland one of its input terminals.

The signal X provided by operational amplifier 40 is applied to a buswire 44. The X-signal provided by source 10 is applied to a bus wire 46.

The signal Y provided by source 12 is coupled to both input terminals 48and 50 of an analog multiplier 52, identical to the multiplier 34. Theoutput terminals 54 and 56 of the multiplier 52 are coupled to anoperational amplifier 58 which provides an output signal Y which issupplied to bus wire 60. The signal Y is supplied to bus wire 62.

A further analog multiplier 64, preferrably identical to the multipliers34 and 52, is provided to develop signals which are functions of X. Thefirst terminal 66 of multiplier 64 is coupled' through a sealingresistor 68 to the X-bus wire 46. A

signal constituting the sum of variable portions of signals X, Y, and Yis coupled to the input terminal 70 of multiplier 64. More particularly,the X bus wire 44 is connected to a potentiometer 72 with the slidecontact thereof being connected through a sealing resistor 74 to theinput terminal 70. Thus, by moving the slide contact of thepotentiometer 72, a selected portion K, of one polarity of the signal Xcan be applied to the terminal 70. Inasmuch as it may be desired for thecomponent X to be either positive or negative, the X bus wire 44 is alsoconnected to an inverter 76 through resistor 78. The output of inverter76 is summed into the signal applied to input terminal 70. The 1 buswire 60 is connected to poten tiometer 80, defining factor K,, with theslide contact thereof connected through a resistor 82 to the terminal70. The Y bus wire 60 is also connected through resistor 84 to theinverter 76 to thus permit either polarity Y signal to be applied to theterminal 70. The Y-bus wire 62 is connected to potentiometer 86,defining factor k:,, with the slide contact thereof being connected toterminal 70 through a resistor 88.

The analog multiplier 64 will produce currents at the output terminals90 and 92 thereof whose difference is related to the function K,X+K,XY+K XY. The terminals 90 and 92 are connected to the input terminalsof an operational amplifier 94. Additional correction signal componentsare summed with the function produced at the output of the multiplier64. More particularly, the X-signal available on bus wire 46 is appliedto potentiometer 96, defining factor K with the slide contact thereofbeing connected through resistor 98 to terminal 90.

The Y-bus wire 62 is connected through resistor 100 to the slide contactof potentiometer I02, defining factor K,, connected between terminals 90and 92. Similarly, the Y bus wire 60 is connected through resistor 104to the slide contact of potentiometer 106, defining factor K connectedbetween terminals 90 and 92. Additionally, the X bus wire is connectedthrough resistor 108 to the slide contact of potentiometer 110, definingfactor K-,, connected across terminals 90 and 92. Still further, apositive potential source is connected through resistor 112 to the slidecontact of potentiometer 114, defining factor K,,, connected acrossterminals 90 and 92.

In view of the explanation thus far, it should be apparent that theoperational amplifier 94 will produce the function depicted in FIG. 1 atits output terminal. That is, the output of operational amplifier 94will consist of the original positioning signal X together with severalcorrection components, each including a variable K term whose magnitudeis determined by one of the aforementioned potentiometers. The compositeoutput signal provided by amplifier 94 constitutes the cor rectedX-defiection signal which is applied to the deflection means 14.

It should be appreciated-that each of the potentiometers thus fardiscussed can vary the magnitude of one of the signal correctioncomponents in the corrected deflection signal to thus enable precisecorrection of the beam position. For identification purposes, thefollowing listing identifies the K terms controlled by each of theindicated potentiometers:

Potentiometer K Term Potentiometer 72 K, Potentiometer 80 K,Potcntiorneter 86 K, Potentiometer 96 K, Potentiometer 102 K.Potentiometer 106 K. Potentiometer K, Potentlometer 114 K,

a product signal comprised ofa Y term which is supplied to anoperational amplifier 122 which yields the Y-positioning signal togetherwith several correction signal components as illustrated in FIG. 1.

As should be appreciated from the foregoing, inclusion of the variouspotentiometers illustrated in FIG. 1, enables each of the correctionsignal components to be individually adjusted'to thus enable atechnician to precisely compensate out any causes of beam-positioningerrors. It is pointed out that although the outputs of amplifiers 94 and122 illustrated in FIG. 1 are shown as being connected to the major X-and Y- deflection means, the apparatus of FIG. 1 can also be employed toeliminate minor positioning errors, as is discussed in the aforecitedpatent application.

Attention is now called to FIG. 2 of the drawing which illustrates apreferred analog multiplier embodiment suitable for use in the apparatusof FIG. 1. The multiplier of FIG. 2 is comprised of first and seconddifferential devices, for example, differential amplifiers. Moreparticularly, a first differential amplifier DAI is provided which iscomprised of a pair of similar transistors 01A and Q18. The seconddifferential amplifier DA2 is comprised of similar transistors 02A and028.

The emitters of transistors Q2A and 02B are connected together and to anessentially constant current source 128 providing a signal 21 Thecurrent source 128 can comprise a very high resistance 130 connected toa source of negative DC potential. The collector of transistor 02A isconnected to the base thereof and similarly the collector of transistor02B is connected to the base thereof. The base of transistor 02A isconnected directly to the base of transistor QlA and the base oftransistor Q2B is connected directly to the base of transistor 018. Thebases of transistors Q13 and Q23 are connected to a source of DCreference potential, e.g., ground.

In accordance with the invention, first and second analog input currentsI, and 1;, are respectively applied to the multiplier input terminals132 and 134. Input terminal 132 is connected through a resistor 136 anda biasing offset circuit 138 to the commonly connected emitters oftransistors 01A and 01B. The biasing circuit 138 is comprised ofresistors 140 and 142, respectively, connected between the commonemitter connection of transistors 01A and 01B and different sources ofDC bias. The purpose of the biasing circuit is to permit circuitoperation with input signals I of ether polarity. The input terminal 134is connected through a resistor 144 and a biasing resistor 145 to thebase of transistor Q2A. The purpose of resistor 145, as will be seen, isto provide a current I, in the absence of an input current 1 and to thuspermit the circuit to operate with either polarity of current 1 As willbe demonstrated hereinafter, the multiplier of FIG. 2 develops theproduct of the analog input signals I and I applied thereto andrepresents the product as the difference between the collector currents1,, and 1 in transistors 01A and 018 respectively. Mathematically, itwill be shown that I,,=I,,I =I -I /I The principle of operation of themultiplier circuit of fig. 2 is based on the concept of establishing acurrent distribution ratio in the paths of the differential amplifierDA2 which is dependent on the magnitude of the current 1 The totalcurrent through the paths of differential amplifier DA2 is fixed at 2 1by the constant current source 128. By interconnecting the bases oftransistors 02A and 02B directly to the bases of transistors 01A and Q18respectively, the same current distribution ratio will be established indifferential amplifier DAl as is established in differential amplifierDA2. The total current flow through the paths of differential amplifierDAl, however, is determined by the analog input signal 1 and accordinglythe difference in the collector current 1,, and I, is proportional tothe product of input signals I 1 and 1 In order to better understand theoperation of the multiplier of FIG. 2, consider initially that thecurrent I: is equal to zero. Accordingly, the constant current 2 I willdivide evenly between the transistors 02A and 02B so that eachtransistor handles a current I The introduction of the'analog inputcurrent 1 to the collector of transistor Q2A increases the collectorcurrent of transistor 02A to I +l,. The collector current in transistor02B will then be represented by I I,. The difference in the collectorcurrents through transistors 02A and 02B forced by the introduction ofthe analog input current I, will establish voltages at the bases oftransistors 02A and 02B at a level required to produce the indicatedcurrent division. Since the base voltages at transistors 01A and 01B arethe same as the base voltages established at transistors 02A and 023,the analog input current I, furnished to the emitters of transistors 01Aand 018 will divide in the same ratio. The change in collector currentsI or 1,, from I,/2 is thus proportional to 1 and the proportion beingthe same for any 1,, the current change must be proportional to theproduct I,XI,. The difference between collector currents 1, and I, canbe readily sensed by a differential operational amplifier with areasonable common mode rejection capability as shown in FIG. 1. Thus theoperational amplifier output represents the product of the two inputcurrents and a factor dependent on I The operation of the multiplier ofFIG. 2 can be more rigorously demonstrated by the following brief andsomewhat approximate mathematical treatment.

In a pair of similar transistors with common emitters supplied currentfrom a high impedance source, let 1 be the sum of the collector currentsand 1,, be the difierence between the collector currents. Now forjunction transistors over a considerable current range, the collectorcurrent is an exponential function of the base to emitter voltage, thusfor either transistor of the pair Where VBEO is the base to emitterpotential required to make the collector current equal to one-half thesum, or in other words, to make both collector currents equal.

One collector current is represented by [(I,/2)+(I,./2)] while the otheris represented by [(I /2 )(I,,/2

The ratio of the two currents, i.e., (I,,+I,)/( I,,I,,) is equal to I,-iI K(VAVBE0A) I. 1,, ,K vBEB-BVEW2 which can be expressd as: 2) L i I e1,, For convenience, let the exponent of e in equation (2) be denoted by2p, then equation (2) may be written (IL is seen to have two components,the VBEO terms representing the offset between transistors and the VBEterms representing the actual potential difference between bases) thus;e p 2 Solving for 1,,

This relation holds for both transistor pairs in the multiplier. Becauseof the collector to emitter feedback the second differential amplifierDA2 develops base to base voltage which is expressed by p. arc tanh xwhere x (2I /2I )=(I /I The first differential amplifier DAl thenproduces an output 1,,

given by AW Equation (5) shows that the product signal 1,, is afunctionof three variables, i.e., I], I and 1 I, and I of course constitute theanalog input factors. Equation (5) stggests that i f I is also varied,maintaining the Ik/ZI ratio, division by I is achieved. This may be doneby controlling voltages across fixed resistors so that the voltages varybut remain in the same proportion. Inclusion of this division capabilityis significant inasmuch as it enables gain to be remotely controlled.

From the foregoing, it should be appreciated that an improved apparatushas been disclosed herein for processing CRT beamtpositioning signals todevelop corrected deflection signals for application to the deflectionmeans of a CRT. Additionally, a very useful analog multiplier circuithas been disclosed which is particularly suited for use in the apparatusfor correcting the beampositioning signals.

I claim:

1. Analog multiplication means comprising:

a first differential device including first and second energy means forestablishing a total energy flow through said first device first andsecond flow paths;

means for establishing a ratio of flow through said first device firstand second flow paths quantitatively related to a first analog inputquantity;

a second difierential device including first and second energy flowpaths;

means for establishing a total flow through said second device first andsecond flow paths quantitatively related to a second analog inputquantity; and

means for establishing the same ratio of flow through said second devicefirst and second flow paths as is established through said first devicefirst and second flow paths by said first analog signal.

2. An analog multiplier comprising:

a first difierential amplifier including first and second currentcontrol devices;

constant current means for establishing a constant total flow throughsaid first differential amplifier first and second current controldevices;

a first analog signal source coupled to said first differentialamplifier for establishing a ratio of current flow through said firstand second current control devices thereof, quantitatively related tosaid first analog signal;

a second differential amplifier comprised of first and second currentcontrol devices;

a second analog signal source;

means coupling said second analog signal source to said seconddifferential amplifier first and second current control devices forestablishing a total current flow therethrough quantitatively related tosaid second analog signal; and

means for establishing the same current distribution ratio between thefirst and second devices of said second differential amplifier as isestablished between the first and second devices of said firstdifferential amplifier by said first analog signal.

3. The analog multiplier of claim 2 wherein each of said seconddifferential amplifier first and second current control devicescomprises a transistor having an emitter, a collector, and a base; and

means coupling said second analog signal source to the emitters of saidsecond differential amplifier current control devices. 4. The analogmultiplier of claim 3 wherein each of said first differential amplifierfirst and second current control devices comprises a transistor havinganemitter, a collector, and a base;

means coupling said constant current means to the emitters of said firstdifferential amplifier current control devices;

means coupling said first analog signal source to the base of said firstdifferential amplifier first current control device; and

means respectively connecting said first differential amplifier firstand second current control device bases to said second differentialamplifier first and second current control device bases.

5. The analog multiplier of claim 2 including means for selectivelyvarying the current magnitude established by said constant currentmeans.

1. Analog multiplication means comprising: a first differential deviceincluding first and second energy flow paths; means for establishing atotal energy flow through said first device first and second flow paths;means for establishing a ratio of flow through said first device firstand second flow paths quantitatively related to a first analog inputquantity; a second differential device including first and second energyflow paths; means for establishing a total flow through said seconddevice first and second flow paths quantitatively related to a secondanalog input qUantity; and means for establishing the same ratio of flowthrough said second device first and second flow paths as is establishedthrough said first device first and second flow paths by said firstanalog signal.
 2. An analog multiplier comprising: a first differentialamplifier including first and second current control devices; constantcurrent means for establishing a constant total flow through said firstdifferential amplifier first and second current control devices; a firstanalog signal source coupled to said first differential amplifier forestablishing a ratio of current flow through said first and secondcurrent control devices thereof, quantitatively related to said firstanalog signal; a second differential amplifier comprised of first andsecond current control devices; a second analog signal source; meanscoupling said second analog signal source to said second differentialamplifier first and second current control devices for establishing atotal current flow therethrough quantitatively related to said secondanalog signal; and means for establishing the same current distributionratio between the first and second devices of said second differentialamplifier as is established between the first and second devices of saidfirst differential amplifier by said first analog signal.
 3. The analogmultiplier of claim 2 wherein each of said second differential amplifierfirst and second current control devices comprises a transistor havingan emitter, a collector, and a base; and means coupling said secondanalog signal source to the emitters of said second differentialamplifier current control devices.
 4. The analog multiplier of claim 3wherein each of said first differential amplifier first and secondcurrent control devices comprises a transistor having an emitter, acollector, and a base; means coupling said constant current means to theemitters of said first differential amplifier current control devices;means coupling said first analog signal source to the base of said firstdifferential amplifier first current control device; and meansrespectively connecting said first differential amplifier first andsecond current control device bases to said second differentialamplifier first and second current control device bases.
 5. The analogmultiplier of claim 2 including means for selectively varying thecurrent magnitude established by said constant current means.